1. Field of the Invention
The present invention relates to a bi-directional optical-amplifier module (OAM) for compensating optical signal loss caused by optical fibers or optical components in an optical transmission system, and more particularly to a multi-stage bi-directional OAM that is capable of suppressing the undesirable multiple reflections in an optical communication system.
2. Description of the Related Art
In a bi-directional optical transmission system, one or more multiplexed optical signals are transmitted in the opposite direction through a single optical fiber. As such, this type of bi-directional transmission system has advantages in that it provides an efficient way of utilizing optical fibers to increase transmission capacity and reduce optical non-linearity in an optical fiber. However, a degradation in optical signals may result from the multiple-reflected light that is generated by Rayleigh-back scattering in the optical fiber and the optical reflection in the optical elements. If the optical amplifiers were used, a further degradation of optical signals may occur due to the amplification of the reflected light. In order to minimize the degradation in optical signals, it is necessary to limit the gain of the optical amplifiers. Thus, it is desirable to use an optical amplifier capable of efficiently amplifying optical signals, while suppressing the multiple reflection of the optical signals.
FIGS. 1a to 1d are schematic diagrams illustrating the OAMs used in the conventional bi-directional optical-transmission systems. The optical-amplifier modules illustrated in FIGS. 1a to 1d propose various configurations for suppressing the multiple reflected light.
The configuration of the bi-directional OAM shown in FIG. 1a is described in U.S. Pat. No. 5,815,308 (entitled “Bi-directional Optical Amplifier). As shown in FIG. 1a, a frequency-tunable-reflection attenuator (FTRA) 110 is interposed between the bi-directional optical amplifiers BOA1 and BOA2 to suppress the multiple reflection of optical signals. The FTRA 110 includes a directional coupler (DC), two optical-band-pass filters (OBPF) 112a and 112b respectively having different pass bands, and two isolators (Iso) 114a and 114b. The function of FTRA110 is to attenuate the reflected light caused by the isolators 114a and 114b or the OBPFs 112a or 112b. Each of the bi-directional optical amplifiers BOA1 and BOA2 includes an erbium-doped fiber amplifier (EDF), a pump-laser diode (pump LD), and a wavelength-division multiplexer (WDM) for applying pumped light to the EDF.
The configuration of bi-directional OAM shown in FIG. 1b is described in C. H. Kim and Y. C. Chung, 2.5 Gb/s×16-Channel Bi-directional WDM Transmission System Using Bi-directional Erbium-doped Fiber Amplifier Based on Spectrally-Interleaved-Synchronized Etalon Filters, IEEE Photon. Technol. Lett., Vol. 11, No. 6, pp. 745-747, June 1999. The module includes a pair of two-stage uni-directional amplifiers coupled together by optical circulators (Cir). In this module, the suppression of multiple-reflected light is achieved by the optical circulators along with synchronized etalon filters of different pass bands, each arranged at mid-stage of the associated two-stage uni-directional amplifier between two EDFs included in the two-stage uni-directional amplifier.
The configuration of the bi-directional OAM shown in FIG. 1c is described in S. Radic, A. Srivastava, T. Nielsen, J. Centanni, and C. Wolf, 25 GHz Interleaved Bi-directional transmission at 10 Gb/s, in Proc. Optical Amplifier and Their Applications '2000, PD7, 2000. The module includes a pair of two-stage uni-directional amplifiers coupled together by wavelength interleavers (IL). In this module, the suppression of multiple-reflected light is achieved by the wavelength interleavers along with isolators respectively provided at the uni-directional optical amplifiers (UOA) included in each of the two-stage uni-directional amplifiers.
The configuration of bi-directional OAM shown in FIG. 1d is described in U.S. Pat. No. 6,018,408 (entitled “Bi-directional Optical Telecommunication System Comprising a Bi-directional Optical Amplifier). In this module, optical waves, which travel bi-directionally, are separated from each other by a wavelength-selective coupler (WSC), then coupled together by another WSC so that they can travel in the same direction. The resultant signal is then amplified by a uni-directional optical amplifier. The output from the uni-directional optical amplifier is split by another WSC into two signals, which are applied to different WSCs, respectively, so that they are bi-directionally traveled. This procedure is indicated by the dotted line arrow in FIG. 1d. The optical signal first passes through a first WSC 141. The optical signal emerging from the first WSC 141 is reflected by a second WSC 142, then applied to a uni-directional optical amplifier 150. The optical signal outputted from the uni-directional optical amplifier 150 is reflected by a third WSC 143, then sent to a fourth WSC 144, which enables the optical signal to travel in the right direction. In this module, the suppression of multiple-reflected light is achieved by the WSCs along with an isolator provided at the uni-directional optical amplifier.
Where it is desirable to increase the bit rate of the channel or the number of multiplexed channels for an increase in the capacity of a bi-directional optical-transmission system, a dispersion-compensating fiber (DCF) and a gain-flattening filter should be used. Generally, such elements are arranged at the mid-stage of a multi-stage optical amplifier in order to minimize a reduction in signal-to-noise ratio. However, it is difficult for those elements to be effectively incorporated in the conventional OAMs, as shown in FIGS. 1a to 1d. 
For example, as shown in FIG. 1a, optical signals, which travel bi-directionally, can be simultaneously amplified by each of the bi-directional optical amplifiers BOA1 and BOA2. However, if a dispersion-compensating fiber exhibiting an increased Rayleigh-back scattering is used, it is necessary to provide a dispersion-compensating fiber at the FTRA 110. That is, separate mid-stage elements must be used for the respective travel directions of optical signals. In addition, as the bi-directional optical amplifiers BOA1 and BOA2 used in the optical-amplifier module are configured without using any isolator, the undesirable lasing or other unstable phenomena may occur. For OAM of FIG. 1b or 1c, separate two-state uni-directional optical amplifiers are used for the respective travel directions of optical signals. For this reason, it is necessary to use separate mid-stage elements for the respective travel directions of optical signals. Furthermore, as shown in FIG. 1d, although it is possible to amplify the bi-directionally-traveling optical signals while achieving a compensation for color dispersion, the signals may be degraded due to the nonlinearity of the DCF as the bi-directionally-traveling optical signals are transmitted in the same direction in the DCF.
As mentioned above, the bi-directional optical transmission technique for bi-directionally transmitting optical signals using a single optical fiber is an efficient scheme to increase the capacity of an optical-transmission system or optical communication network through a single optical fiber. However, this technique has a problem in that the optical-transmission system or optical-communication network may have a limited performance due to the multiple-reflection of optical signals that is caused by Rayleigh-back scattering or various reflections occurring in the optical fiber. In particular, where the optical-transmission system or optical-communication network uses an optical amplifier, the amplification and accumulation of multiple-reflected lights are generated. As a result, the gain of the optical amplifier is limited.